Category: Windows Clustering

Cluster terminology – What “Active/Active” actually means

As a follow-up to my last entry (attempting to clear up some Windows Clustering terminology), I’ve found an article that makes another distinction that I forgot to include – the difference between an active/passive and an active/active cluster:

The misconception of active/active clustering (a la AirborneGeek.com)

The understanding among those new to cluster seems to be that a/a vs. a/p is a licensing question, and then if you’re licensed for it, you just turn it on. In reality, it really just describes whether you have clustered services living on only one node or split between both nodes (during normal operation – during a cluster failover, any cluster might be active/active for a short period of time. Or, I suppose, your cluster is active/active if your quorum drive lives on the opposite node from your clustered service). There’s no load-balancing involved in clustering at all – at any time, only one node owns a particular resource, and only that node is responding to client requests for that resource.

In SQL Server 2012 AlwaysOn, the new high-availability feature, the SQL Server service is running on both cluster nodes, but client access (through the “Availability Group”) is controlled by the cluster service. That means that all clients making a connection go first to the active server, and then the SQL Service there might send them to get their data from one of the other nodes (it’s worth reiterating here that, in AlwaysOn, SQL Server isn’t clustered, but the SQL services operate independently on each node).

Clearing up Windows Cluster terminology

I wanted to clear up some terminology around Windows Clusters that seems to cause a bit of confusion. I’ve stumbled across a few questions on StackOverflow and Experts-Exchange that seem to have some basic confusion around “clustering servers” and “how to install an application to a cluster”, and I’m hoping to set a few things straight.

  1. There’s really no such thing as a clustered server. Servers can have clustering enabled and configured, but the servers themselves aren’t really clustered – they’re just set up to enable clustered applications. When servers are part of a cluster, they still do all their thinking on their own, including running their own applications, services, and tasks, without the other servers in the cluster even being aware.
  2. You don’t cluster servers, you cluster applications and resources. Once servers have had clustering installed and are configured, you can cluster an application or a resource. This clustering is really just telling the cluster manager that you want it to control which server clients talk to when they want to access the resource. The cluster manager ensures that the application (or service or resource) is running on only one node at any given time, and to the extent it’s able, it ensures that it’s always running (watching for a failure and bringing the resource online on another node and then directing clients to that node instead).
  3. Applications don’t have to be “cluster-aware” to be clustered. I work mostly with SQL Server, which is cluster-aware, but applications you cluster don’t need to be. You can cluster any service, or resource on a server by just adding it to the cluster manager – the cluster manager will ensure it only runs on one server at a time, not allowing it to start on other nodes. For example, we use a monitoring tool that runs as a service – we installed the service on each cluster node and then added to the cluster manager – it now can be failed back and forth between nodes as a clustered resource, so it’s always online, is failure-resistant, and shares a segment of the HKLM in the registry between nodes – all without being explicitly cluster-aware.
  4. SQL Server doesn’t need to be clustered when it’s installed on a cluster. While you can install a clustered instance of SQL Server (which automatically registers everything with the cluster manager), you can also install stand-alone instances of SQL Server (or any other application) on a cluster. That’s actually how a new feature in SQL 2012 – AlwaysOn – works: You install a non-clustered instance of SQL Server on different cluster nodes, and then you let the cluster manager coordinate client connections to the SQL Servers, but they still operate independantly and replicate their data between each other.

Hopefully this clears things up and doesn’t lead to more confusion. When I first started working with clustering, I had the impression that setting up a cluster caused the servers to act as one and share all their resources, but that understanding led to a lot of confusion when it came time to set something up or troubleshoot an issue. With the understanding that “clustered servers” are really just servers with clustered resources, and not actually clustered themselves, hopefully it will simply things!

Changing SQL Server data file locations in clustered instance using service SIDs

Recently I added a second LUN to a clustered instance of SQL Server to isolate the logs files (and another for the TempDB) and I mounted them to an empty NTFS folder as I’ve done before, but when I started SQL Server, I recieved the following error:

CREATE FILE encountered operating system error 5 (Access is denied.)
while attempting to open or create the physical file
'X:\MSSQL10_50.INSTANCENAME\TempDB\tempdb.mdf

The error means that SQL Server doesn’t have NTFS rights to the location of the TempDB, but when I tried to add those rights, the permissions weren’t granted to the domain proxy account as I’d expected, but were instead granted to the service SID account, MSSQL$InstanceName. I attempted to grant the permissions to this account at the new location, but couldn’t get it to resolve to an actual account. What finally worked was:

  1. In the permissions change dialog box, change the “Location” from the domain to the local machine (even though it’s a cluster and your using a domain account to run the service)
  2. In the text box, type “NT Service\MSSQL$INSTANCENAME” and click “Check Names”

Even though that appears to be a local account, it will resolve properly on all the cluster nodes involved. This step, as opposed to using the domain proxy account the service is running as, was necessary because (during the initial SQL Server setup process) I’d selected to use the proxy account SID to host permissions rather than a domain group. The better choice permissions-wise, but the source of some confusion!

Clustering walkthrough for SQL Server 2008 on Windows 2008

I recently stumbled across a great walk-through for clustering SQL Server on newer versions of Windows. It’s really thorough – everything from setting up iSCSI (in this case, to simulate a shared disk when it’s physically attached to one node – not ideal, but lets you test the walk-through), adding the required server roles, preparing the servers, and then a walkthrough of every screen in the SQL installation process. Thanks to the writer of this awesome blog!

http://dbperf.wordpress.com/2010/07/10/walkthrough-cluster-setup-sql-win-2008/

Slow SQL Server queries over a linked server from Windows Server 2008

We recently ran into an issue where linked server queries where sometimes taking significantly longer than they should have – queries that executed in under a second on the source server took two minutes when executed from a remote SQL Server over a linked server connection. We assumed it was something to do with our Windows Failover Cluster network connections, but it was happening on non-clustered servers as well.

After some digging, we discovered that it happened when the source server was Windows Server 2008 (or 2008 R2), regardless of what OS the remote linked server was running, and the issue wasn’t tied to any particular version of SQL Server. As it turns out, it related to everybody’s least favorite network setting, TCP Auto-Tuning – disabling it on the affected source servers resolved the problem immediately, and the queries returned instantly again.

To remedy the situation, follow these steps:

  1. On the source SQL Server (the one executing the query, not the linked server), open an elevated command prompt (Right click on CMD.EXE and click “Run as Administrator”)
  2. To disable TCP Auto-tuning, run the following command:
    netsh int tcp set global autotuninglevel=disabled
  3. To disable Windows Scaling Heuristics (another feature that can speed up clients, but is no good for servers), run:
    netsh int tcp set heuristics disabled
  4. This will automatically take effect after Windows has the chance to refresh some network communications – you can either wait about an hour, or, if you’d prefer it take effect immediately, restart the SQL Instance. A reboot isn’t necessary for this setting to take effect.

I’m not sure why these settings are on by default for servers – they seem to speed up internet browsing and other client-type activities (though there’s no shortage of complaints online about them grinding Windows Vista/7 clients to a halt as well), but can cause big problems on servers.